Medical diagnostic x-ray machines typically comprise a source of x-rays, a flouroscopic, electronic, or photographic x-ray detector, and means for supporting a patient's anatomy of interest between the source and detector. A collimator must be interposed between the source and detector, and usually is positioned between the source and the patient, to confine the radiation to a beam of precise size and shape. The collimator limits the exposure of the patient to x-rays as much as possible, consistent with the aim of disclosing the patient's anatomy of interest.
Several types of collimators are known in the art. In plate collimators, one or more apertured plates disposed transversely to the direction of propagation of radiation are interposed in the beam of radiation. The radiation striking the plate beyond the margin of the aperture is deflected or absorbed. The beam projected through the aperture has a cross-sectional shape similar to that of the aperture and a size or diameter controlled by the position of the focal point of the radiation, the position and size of the aperture, and the position of the detector.
In some plate collimators, the size of the beam has been regulated by an iris diaphragm or similar structure. An iris diaphragm is cumbersome, since its leaves must be made of lead and thus are thick and heavy. An iris diaphragm also is not capable of producing a truly circular aperture, particularly for very small openings, and has a limited range of diameters. An iris is difficult to design if an unusual cross-sectional configuration for the x-ray beam is desired, such as a keyhole shape or the shape of an organ to be studied.
In other known plate collimators a plate (or series of plates) having apertures of various sizes has been interposed in the beam. These collimators are undesirable because the diameter of the pattern projected on the detector cannot be continuously adjusted.